Antitoxins act to neutralize the effects of toxins on specific cells; production of protective antibodies following immunization with toxoid vaccines containing a Diphtheria component is required for release of the vaccine lot. Currently, a second animal test is employed to evaluate the efficacy of the animal response to the injected vaccine. This research has focused on replacing this second animal test with an in-vitro cell test. Results of the six laboratory collaborative study were presented in December, 1997 at the FDA Science Forum. In this collaboration, there was a 95 % correlation of animal and cell tests participants. Because of limited animal and financial resources, efforts to investigate the 5% difference have been delayed. This year, this assay was investigated as a potential alternative method for determining the neutralizing activity of Diphtheria antibodies in Immune Globulin preparations, and in sera from immunized children. Preliminary studies of a limited number of Immune Globlins and patient samples suggest that the current methods of assaying diphtheria antibody activity may not correlate with toxin neutralizing activity in animals. Investigation is continuing both CBER and University of Maryland laboratories as time and funding permit.